Robot Power Systems — Batteries, BLDC Drivers, Regen

See also (Tier 3 family index): Battery Chemistries

1. At a glance

Every robot is a power system before it is a kinematic chain. The bus voltage you pick, the chemistry you pick, the driver you pick, and the way you handle deceleration energy together determine the robot’s runtime, mass, thermal behaviour, safety profile, and, in industrial deployments, whether it is certifiable at all. Mechanical engineers underspecify it constantly because a battery looks like “a box that delivers volts” — it is not. It is a chemical reactor with a state of charge, a temperature gradient, an internal resistance that triples in the cold, an aging trajectory, and a failure mode that ends with the building on fire.

The system chain is invariant across robot classes: cells → pack + BMS → DC bus → DC/DC rails + motor drivers → motors → kinetic energy → (regen back into the bus or burned in a brake resistor) → charging. Sizing one element without the others is the same mistake covered in [[Robotics/motors-electric]] for the motor–gearbox–driver chain. You design the whole loop, including the energy return path, or it bites you in deployment.

Dominant chemistries by application class (2024–2026 industry state):

LiPo (Lithium-Polymer pouch) — high C-rate, the only practical choice for multirotors and racing drones ([[Robotics/multirotor-design]]).
Li-ion cylindrical (18650 / 21700) — high energy density, rigid form factor; the workhorse of laptops, e-bikes, humanoid robots (Tesla Optimus, Figure 02, Apptronik Apollo).
LiFePO4 (Lithium Iron Phosphate) — highest safety, longest cycle life, lower energy density; dominates indoor AGV / AMR (MiR, Fetch, Otto), industrial floor cleaners, and any robot that must charge unattended in a workplace.
NMC pouch (large-format) — EV-style; high energy density; appearing in heavy mobile platforms, eVTOL, and humanoid integrated packs.
Solid-state (emerging) — QuantumScape, Solid Power, Toyota; 400+ Wh/kg target, automotive timeline 2026–2028, not yet a real robotics part.

On the drive side, every modern robot of any sophistication is FOC-driven BLDC / PMSM (see [[Robotics/motors-electric]] §2 for the Park/Clarke math and §3 Example C for sizing); steppers and brushed DC survive in low-cost niches but the BLDC + FOC + integrated current loop is the default. The driver is itself a power-electronics converter (three-phase H-bridge inverter, see [[Engineering/power-electronics]] §2 for switch-mode operation) and the same volt-second / charge-balance discipline applies.

The first ask, before any cell is ordered: what duty cycle, what runtime, what charge cadence, what safety envelope, what mass budget, and what bus voltage does the joint hardware actually want. Skip the ask and the BOM is wrong.

2. First principles

Cell terminal equation. Under load, a cell’s terminal voltage is:

V_terminal = V_OC(SoC) − I · R_int

V_OC(SoC) is the open-circuit voltage curve of the chemistry (sigmoid-ish, ~3.0–4.2 V for Li-ion / LiPo, much flatter ~3.0–3.4 V for LiFePO4). R_int is the internal resistance — a few mΩ for a fresh 18650 at 25 °C, rising sharply with age and falling temperature. R_int triples at −10 °C and roughly doubles by end-of-life (SoH 80 %). Voltage sag under high current is the single most-misdiagnosed symptom in field robots: the cell still has capacity, but its delivered voltage drops below the inverter’s operable range.

Capacity, energy, C-rate. Capacity C is in ampere-hours (Ah). Energy E = C · V_nom in watt-hours (Wh). C-rate is the discharge current expressed as a multiple of capacity: 1C = full discharge in one hour, 10C = six minutes, 30C ≈ two minutes. A 5 Ah LiPo rated 30C continuous delivers 150 A continuous; rated 60C burst delivers 300 A for a few seconds. Hobby ratings are aggressive marketing — derate by 30 % for engineering work.

Energy density vs power density. Wh/kg measures runtime per kilogram; W/kg measures peak power per kilogram. They trade off. A high-C-rate LiPo (drone) optimises W/kg at the cost of Wh/kg. A high-energy Li-ion 21700 (laptop, humanoid) optimises Wh/kg at the cost of W/kg. Picking the wrong axis is the most common chemistry error: drone designers buy energy cells and brown out under throttle, AGV designers buy power cells and run short.

Discharge curve geometry. A Li-ion discharge curve has three regions: a fast initial drop (top 5 % SoC), a long, nearly-linear plateau (95 %–20 %), and a knee plus collapse (below 20 %). The plateau is what gives Li-ion its usable range; the knee is where the BMS must cut off to avoid permanent damage. LiFePO4 is much flatter — a virtue for runtime predictability but a curse for SoC estimation, because terminal voltage barely changes over 80 % of the cell’s energy and Coulomb counting becomes mandatory.

Cycle life and state of health (SoH). SoH is the ratio of present full-charge capacity to nameplate, expressed as %. Conventional “end of life” is SoH = 80 %. Cycle counts to 80 % SoH depend strongly on depth of discharge (DoD): a Li-ion cycled to 100 % DoD typically reaches SoH 80 % in 400–700 cycles; cycled only to 80 % DoD, in 1500+ cycles; cycled to 50 % DoD (the laptop-friendly range), 3000+ cycles. LiFePO4 doubles those numbers across the board. The single biggest lifetime win for any robot is to design the duty cycle so the pack lives at 30 %–80 % SoC and only sees the full 0–100 % swing at deliberate “depot-charge” intervals.

Coulombic efficiency. η_coul = charge_out / charge_in per cycle. Mature Li-ion / LiFePO4 cells are 99 %+; the 1 % loss is irreversible plating / SEI growth. Coulombic efficiency is not energy efficiency: energy efficiency η_energy = η_coul · (V_discharge_avg / V_charge_avg) is typically 90–95 %.

BMS responsibilities. A Battery Management System is not optional; it is the safety boundary. Six core functions:

Per-cell voltage monitoring at the parallel-group (“p-group”) level; trip on over-voltage (OVP) or under-voltage (UVP).
Per-cell or per-group temperature sensing; trip on over-temperature charge (OTC), discharge (OTD), or low-temperature charge (LTC, critical — charging a Li-ion below 0 °C causes lithium plating).
Pack current sensing, with over-current trip (OCT) and short-circuit detection (SCD).
Active or passive cell balancing during charge.
State of charge / state of health estimation (Coulomb counting + voltage curve + Kalman or EKF; see [[Robotics/bayesian-estimation]]).
Communication to host controller, typically CAN (open: SMBus, J1939; proprietary: vendor-specific) or RS-485 / Modbus for industrial AGV.

A robot without a real BMS is a fire waiting for an ambient temperature swing.

Power electronics in the drive chain. Three converter stages live on a typical robot:

Battery → DC bus — often direct (pack voltage is the bus) for SELV systems. On large or multi-pack robots, a synchronous buck-boost levels packs.
DC bus → motor phases — the three-phase H-bridge inverter (six switches, gate driver, current sense, dead-time). FOC firmware closes the current loop here.
DC bus → auxiliary rails — buck converters for 24 V, 12 V, 5 V, 3.3 V compute / sensor rails. Isolated when the compute domain must be safely separated (cobot E-stop chain).

Each is a [[Engineering/power-electronics]] problem with hard-switched MOSFETs, ringing, EMC, and thermal soak. The motor inverter is the most stressed: its switch node sees the full pack voltage at edges with 100 V/ns dv/dt and is the dominant EMI source on the robot.

Regenerative braking. When a motor under FOC decelerates, the back-EMF still exceeds zero and the q-axis current reverses sign: the inverter operates as a synchronous rectifier and pumps current back into the DC bus. Energy is recovered. Where does it go?

Into the battery (the wanted path), if the battery can absorb it.
Into bus capacitance (a brief voltage rise — the “bus pump”).
Into a brake resistor (a dump resistor switched onto the bus when voltage exceeds a threshold), when the battery cannot accept the current.

For Li-ion, accept-rate is limited at high SoC (a full pack at 4.2 V/cell will trip OVP on a regen pulse). For LiFePO4, broader headroom but still bounded. The bus must therefore include either a brake chopper or an actively-limited regen current loop. Neglecting this is the defining failure mode of large mobile robots and AGVs on downhill ramps: bus voltage spikes, the inverter trips, the load falls.

Field-oriented control loop (recap). Three phase currents → Clarke → Park → (i_d, i_q) → two PI loops → inverse Park → inverse Clarke → SVPWM → three duty cycles. i_q is the torque-producing current, i_d is held at zero for surface-PM motors and used for field-weakening above base speed. Current loop bandwidth typically 1–3 kHz; SVPWM frequency 10–40 kHz. The driver IC (ODrive, VESC, Maxon EPOS, Trinamic TMC4671) implements this in hardware or firmware. Sizing the current loop is in §3 Example C.

Bus capacitance and ride-through. The DC bus must include enough capacitance to (a) absorb the high-frequency current pulses drawn by the inverter, (b) smooth pack-level voltage sag, and (c) hold the bus over micro-interruptions (loose connector, contactor bounce). A starting rule: bus capacitance C_bus ≥ I_peak / (f_sw · ΔV_allowed) where ΔV_allowed is the tolerable cycle-by-cycle ripple (typically 1–2 % of bus). At 100 A peak, 20 kHz, 1 % of a 48 V bus (0.48 V): C ≥ 100 / (20 000 · 0.48) = 10.4 mF. Always polymer or hybrid-aluminium with ESR < 10 mΩ. Ceramic decoupling (10× 4.7 µF X7R) close to the inverter MOSFETs absorbs the picosecond-scale switching transients; bulk electrolytic / polymer holds up the slow swing. Mixing both is mandatory above ~200 W.

Pre-charge. Closing a main contactor onto a discharged bus capacitance creates an inrush current of V_bus / R_contactor — easily several kA — that welds the contactor and stresses cells. A pre-charge resistor (e.g., 100 Ω, 100 W) in parallel with the main contactor, switched on first via a small relay, slowly charges C_bus to within 5 % of V_pack; only then does the main contactor close. Standard practice in any pack > 24 V with > 1 mF of bus capacitance.

3. Practical math — three worked sizing examples

Example A — Indoor AMR battery sizing

An indoor warehouse AMR: average draw 12 A from a 24 V bus during driving + lift cycles, target shift runtime 8 hours, charge cadence one fast top-up per shift.

Energy requirement:

E_required = V · I_avg · t = 24 V · 12 A · 8 h = 2304 Wh.

With a 20 % design margin for cold days, end-of-life capacity fade, and unaccounted accessory load:

E_pack = 2304 · 1.20 = 2765 Wh; round to 2880 Wh.

Chemistry choice. LiFePO4: safest for indoor unattended operation, longest cycle life, lowest insurance premium. Energy density ≈ 130 Wh/kg at pack level (after BMS, casing, busbars).

Pack configuration. LiFePO4 cell nominal V = 3.2 V, full V = 3.65 V, empty V ≈ 2.5 V. Bus target 24 V → 8 cells in series (8S) gives V_nom = 25.6 V, V_full = 29.2 V. Capacity at 2880 Wh / 25.6 V = 112 Ah. Standard prismatic cells (EVE LF105, CATL 100 Ah, BYD 105 Ah) at 105 Ah → 8S1P, 2688 Wh. Slightly under target; go 8S × 2 cells parallel of a smaller cell, or accept the 6.5 % shortfall, or step to 120 Ah cells.

Mass. 2880 / 130 = 22 kg pack mass. Volume ≈ 14 L (LiFePO4 is bulky).

Cycle life. LiFePO4 at 80 % DoD ≈ 3000 cycles. At 1 charge per workday, 250 workdays per year → 12 years. Pack outlives the robot.

BMS. 8S LiFePO4 → TI BQ76952 (16S-capable, $3) o na c u s t o mb o a r d, oro ff - t h e - s h e l f D a l y 8 S 100 A BMS f or$ 60. Cell balancing passive (300 mA resistive). Comms over CAN to the AMR’s main controller.

Charger. 29.2 V (3.65 V/cell), 30 A (≈ 0.3 C) CC-CV charger — 1.5 hour fast charge from 20 % to 100 %. Meanwell RSP-1000-27 or similar industrial charger.

Example B — Drone discharge profile (continued from `[[Robotics/motors-electric]]` Example C)

A 1.2 kg quadcopter, four T-Motor MN3508-29 KV700 on 9-inch props, 4S battery (14.8 V nominal). From the motor sizing example, hover power per motor ≈ 60 W → 240 W total at the bus, plus ≈ 20 W flight controller / radio / camera. Hover bus power ≈ 260 W.

Battery pick. Tattu R-Line 4S 1550 mAh 120C: V_nom 14.8 V, E_nom = 14.8 · 1.55 = 22.9 Wh; mass 196 g.

Hover endurance with 85 % usable capacity (LiPo knee + safety margin):

t_hover = 22.9 · 0.85 / 260 = 0.0749 h = 4.5 min.

Tight — typical for an aggressive 5” racer / 9” cinematic-quad balance. For longer endurance, step up to 4S 2200 mAh: 32.5 Wh → 6.4 min hover, mass 270 g. AUW rises to ≈ 1.27 kg, hover power up to ≈ 280 W, true endurance ≈ 5.9 min. Diminishing returns: heavier battery costs hover power that costs endurance.

Peak current check. During an aggressive pull-up, four motors at 1.5× hover current ≈ 90 A from the bus. Battery rated 120C · 1.55 = 186 A burst, 60C · 1.55 = 93 A continuous (derate from datasheet). Margins tight; specify 4S 2200 mAh 95C (210 A burst, 105 A continuous) for headroom.

Voltage sag. R_int of a 4S LiPo at ≈ 4 mΩ/cell × 4 = 16 mΩ. Sag at 90 A = 1.44 V → terminal 13.4 V instead of 14.8 V. Acceptable; below 50 % SoC the sag worsens as R_int rises.

Charging. iCharger or ISDT Q8: 4S balance charge at 1C = 1.55 A in CC-CV mode; ~1 hour. Fast 4C charge cuts to 15 min but halves cycle life.

Example C — BLDC current loop sizing

Match a current-loop bandwidth to a [[Robotics/motors-electric]] Example C drone-style motor: T-Motor U5 (1100 KV, used in larger 13–15” prop frames; we’ll use it generically), 6S bus (22.2 V nominal), 30 A peak phase current. Phase resistance R_phase = 0.1 Ω, phase inductance L_phase = 50 µH (line-to-neutral, typical of low-KV high-pole-count outrunners).

Plant. The d- and q-axis circuits each look like a first-order R-L plant with time constant τ_e = L / R = 50 µs. The corner frequency f_e = R / (2π·L) = 318 Hz.

Closed-loop target. Current loop bandwidth ω_c ≈ 2π · 1500 = 9425 rad/s (1.5 kHz). Rule of thumb (Vedder / VESC, ODrive): ω_c at 5–10× the open-loop electrical pole, well below the SVPWM Nyquist (f_sw / 2; at 20 kHz SVPWM, Nyquist 10 kHz).

PI gains via pole-zero cancellation. Set the PI zero at the plant pole so the closed-loop is first-order with bandwidth ω_c:

K_p = L · ω_c = 50e-6 · 9425 = 0.471 V / A K_i = R · ω_c = 0.1 · 9425 = 942 V / (A·s)

Equivalently the integrator gain is K_i / K_p = R / L = 1 / τ_e — the canonical “set the zero to cancel the pole” recipe used by every current-mode FOC controller (ODrive’s motor.current_control_bandwidth, VESC’s foc_current_kp / foc_current_ki).

Voltage headroom. At 30 A through 0.1 Ω, the resistive drop is 3 V. The motor back-EMF at peak speed (back-EMF constant ≈ 9.55 / 1100 = 8.7 mV·s/rad ≈ 0.91 V/krpm) at 14 000 rpm ≈ 12.7 V back-EMF. Total V required ≈ 12.7 + 3 = 15.7 V at the phases, with overmodulation to 0.95 of bus → V_bus must be ≥ 16.5 V. 6S nominal 22.2 V is comfortable; at 6S empty 21.0 V (3.5 V/cell), still fine; at 6S full 25.2 V, large margin.

Sample rate. Loop sample at SVPWM frequency 20 kHz (every 50 µs). Equivalent zero-order-hold delay 25 µs. Phase margin shrinks; bandwidth target 1.5 kHz preserves > 60° PM.

4. Design heuristics

Chemistry by application:

Drone / multirotor: LiPo (5–30C, 4S–6S typical). LiHV (4.35 V/cell) for the last 5 % runtime if cycle life is sacrificable.
Indoor AMR / AGV / floor scrubber: LiFePO4 first choice. NMC pouch only if mass-critical.
Humanoid: Li-ion 21700 (Optimus, Figure) or NMC pouch (large packs). 18650 obsolete for new designs.
Cobot (fixed base): tethered AC mains via internal DC bus; no battery.
Outdoor / field robot: Li-ion 21700 high-density, or NMC for very large robots. Hydrogen PEM fuel cell for long-endurance (8+ hour) where mass and runtime both matter — Intelligent Energy IE-Power 800 W stack, ~1.5 kWh/kg system.
Underwater / marine: sealed Li-ion pressure pots, or LiFePO4 in pressure-balanced oil-filled enclosures.

Cells in series, cells in parallel. Series (Ns) raises voltage, parallel (Np) raises capacity and lowers R_int. For a target W power level, higher bus voltage means lower current, smaller wires, smaller MOSFETs. The cost: more cells to monitor and balance, higher creepage / clearance demands, and the 50 V SELV (Safety Extra-Low Voltage) line crossed if Ns × V_max > 60 V DC. Above 60 V DC the design enters shock-hazard territory under most national codes — interlocked enclosures, doubled insulation, ground-fault detection. Pick 48 V nominal (13S Li-ion / 15S LiFePO4) as the sweet spot for medium and large mobile robots: maximum power within SELV.

Common pack configurations (see §6 table for energy figures):

4S LiPo (14.8 V) — drones, light hobby robots.
6S LiPo (22.2 V) — larger drones, RC cars, light mobile bases.
7S–10S Li-ion / 8S LiFePO4 (24 V class) — small AMRs, cobot teach pendants on battery.
12S–14S Li-ion / 16S LiFePO4 (48 V class) — medium AMRs, e-bikes, quasi-direct-drive legged robots.
16S–24S Li-ion (60–100 V) — heavy AGVs (above SELV; safety case required).
96S+ NMC pouch (400 V class) — autonomous vehicles, eVTOL.

BMS architecture.

Centralised: one IC monitors all cells via long wiring harness. Simple, cheap, fine up to ~16S. TI BQ7693006, Maxim MAX17853.
Distributed: cell-monitor IC at each module; daisy-chained iso-SPI back to the master. Mandatory above ~24S or in EV-class packs. ADI LTC6811-1 / LTC6813-1 (12S/18S per IC), TI BQ79616, NXP MC33771C.
Wireless: emerging in production EVs (GM Ultium, Tesla rumoured). Removes the iso-SPI harness. Not yet a robotics-grade off-the-shelf solution.

Cell balancing. Passive: each cell has a parallel MOSFET + resistor; the BMS dissipates excess charge from high cells as heat. Cheap, slow (50–300 mA balance current), waste energy. Active: charge is transferred from high to low cells via a capacitor (switched-cap) or transformer (flyback / forward) — 1–5 A balance current, no heat penalty, more complex. For robotics: passive is fine for cobot-class packs charged daily; active pays off in EV and humanoid packs where balance current must keep up with imbalance growth during deep cycles.

DC bus voltage selection — heuristics:

Under 24 V → high current, large wires, MOSFETs in parallel; only used in small (<200 W) robots.
24 V → indoor AMR sweet spot for sub-1 kW; LiFePO4 8S fits naturally.
48 V → 1–5 kW class robots; SELV; legged robotics standard (Unitree Go, MIT Mini Cheetah QDD).
60–100 V → industrial AGV with rated motor drives (heavy lifters); safety case mandatory.
400+ V → autonomous vehicles, eVTOL; outside scope of this note.

Charging strategies.

CC-CV: canonical Li-ion / LiPo charge. Constant current to V_max per cell (typically 4.2 V Li-ion, 3.65 V LiFePO4), then constant voltage until current tails to ~0.05C. ~3–4 hours at 0.3C.
Fast charge: 1–3C, 20–40 min from 20 % to 80 %. Needs active thermal management; degrades cycle life by 30–50 %. The right answer for AGV fleet operations.
Opportunity charging: short top-ups during natural idle pauses (e.g., AMR at a workstation). Keeps pack at 40–80 % SoC where degradation is slowest.
Contactless inductive: wireless wheel-floor coupling (Wibotic, WiBotic 300 W or 5 kW lines; Mojo Mobility). 80–90 % efficient. Eliminates connector wear.
Brush / contact docking: mechanical sprung contacts on the robot mate to floor pads. Cheap, robust, used by Roomba and many AMRs.
Hot-swap pack: two packs, operator (or auto-arm) exchanges them. Used in factory AGV fleets demanding 24/7 uptime.

Brake resistor sizing. Required when peak regen current exceeds what the battery can accept and the brake chopper engages. For a deceleration from v to 0 of a mass m at a deceleration time t_dec: regenerated power P_reg ≈ ½ m v² / t_dec (less drivetrain inefficiency). For a 50 kg AMR at 1.5 m/s decelerating in 0.5 s: P_reg ≈ ½ · 50 · 1.5² / 0.5 = 112 W. Resistor R = V_bus² / P_reg ≈ (29 V)² / 112 = 7.5 Ω. Pick a wirewound 8 Ω, 250 W resistor (Ohmite TGHG, Vishay RWM) to handle thermal soak with margin. For larger or downhill-traversing robots, P_reg can hit kilowatts — size accordingly.

Distribution architecture. Drones use a power distribution board (PDB) with copper-laminate busbars; ESCs solder directly. Mobile robots use a fused distribution panel — main contactor (e.g., Gigavac GX11 or Tyco EV200) for full pack disconnect, fuses per branch (motors, compute, sensors, lighting), each branch fed through a switched DC-DC if it needs a different rail. Compute domain (Jetson, NUC, sensors) on a stable buck-regulated 12 V or 19 V rail isolated from the noisy motor bus to suppress conducted EMI. See [[Engineering/power-electronics]] §3 for the buck design math.

E-stop chain integration. Cobot and AGV designs route a hardwired E-stop loop (a normally-closed series circuit through all E-stop buttons, light curtains, door interlocks) to a safety relay (Pilz PNOZ, Schmersal SRB, Banner GM-FA) which opens the main contactor on any break. The safety chain must be independent of any software (no microcontroller in the loop) and must be rated to ISO 13849-1 PL d or PL e for collaborative robotics. The BMS contactor and the E-stop contactor can be the same physical device if it is sized for both currents and the controllers agree on opening priority. See [[Robotics/safety-standards]] for the full safety-case framework.

Wire sizing and busbar selection. A rule of thumb for chassis wiring: 8 A per mm² of copper cross-section for 30 °C rise in still air; 12 A per mm² with airflow. For a 100 A bus: ≥ 12 mm² (4 AWG) wire. Busbars (laminated copper or aluminium) start being attractive above ~50 A — easier to fix to chassis, lower inductance, easier to fuse. AGV main bus is almost always a busbar; drone PDB is a printed-copper plate with 70 µm or thicker copper.

Connector current ratings. Each connector has both a continuous and a peak rating (and a 1-second short-circuit rating, less commonly published). XT60 is rated 60 A continuous; XT90 90 A; Anderson PowerPole 75 A (Series 30/45) or 175 A (SB175). Never run at more than 80 % of nameplate continuous. Connectors are the most common reliability failure point on a mobile robot — corrosion, fretting, contact resistance growth — inspect quarterly and re-seat or replace any showing > 5 mΩ resistance.

SoC estimation strategies. Coulomb counting (∫i dt) is simple but drifts; voltage-curve mapping is direct but unreliable on flat-curve LiFePO4; Extended Kalman Filter (EKF) on an equivalent-circuit model with V_OC(SoC), R_int(SoC, T), and a 1- or 2-RC dynamic branch is the production-grade answer (Plett vol II). Modern BMS ICs (TI BQ40Z80, TI BQ34Z100) implement Impedance Track or proprietary EKF variants — accuracy ±2 % SoC over the cell lifetime if the impedance reference is updated periodically by full cycles. See [[Robotics/bayesian-estimation]] for the underlying Kalman math.

Charging dock alignment. Final-cm alignment is a perception problem: fiducial marker (ArUco / AprilTag) on the dock + monocular camera, or LiDAR-fitted reflectors, or a magnetic homing sensor. Last 10 mm typically via a compliant spring-loaded contact array that tolerates ±5 mm position error. See [[Robotics/computer-vision-robotics]] for the visual servo loop.

Thermal management at the pack level. A 1 kWh pack delivering 100 A at 5 mΩ R_int dissipates 50 W of waste heat into the cells. For a 22 kg LiFePO4 pack with specific heat ≈ 1100 J/(kg·K), a 1-hour run at that load raises pack temperature by 50 · 3600 / (22 · 1100) ≈ 7.4 °C above ambient with zero cooling. Acceptable for room-temperature operation; under sustained outdoor sun or in an enclosed robot chassis, it stacks with ambient soak and can drive the pack to its 60 °C trip. Cooling options, in order of cost: (a) chassis-airflow convection (free), (b) forced-air fan (~5 W), (c) cold-plate with liquid coolant (~30 W pump, EV-class), (d) refrigerant immersion (research only). Heat must leave through busbars or pouch tabs — internal cell heat flux is constrained by the separator.

Safe charge envelope. Li-ion / NMC: charge between 0 °C and 45 °C, V_max 4.20 V (or 4.35 V for HV), charge current ≤ 1C standard / 3C fast. LiFePO4: charge between 0 °C and 55 °C, V_max 3.65 V, charge current ≤ 1C standard / 3C fast. Below 0 °C, no charge allowed (plating risk); pre-heat the pack first. Above the upper trip, BMS terminates charge. Discharge envelope is broader: −20 °C to 60 °C typical for both chemistries, with capacity penalties at the cold end.

Pack-level safety case (ISO 13849-1 / IEC 62619 / UL 2580). Industrial robot packs deployed in workplaces need a documented safety case covering: cell thermal runaway containment, fault propagation analysis, BMS independence from main controller (single-point-failure analysis), short-circuit + over-current isolation (HRC fuses + redundant contactor), emergency shutdown chain, and labelling per UN 38.3 for transport. Cobot and AGV vendors typically certify to IEC 62619 + UL 2580 + relevant regional EMC; budget 3–6 months and $50–200k for first-time certification.

5. Components & sourcing

Lithium-ion cylindrical cells (2024–2026 inventory):

Panasonic NCR18650B (3400 mAh, 4.9 A continuous) — legacy high-energy, e-bike default.
Panasonic NCR18650GA (3450 mAh, 10 A) — Tesla Model S 85 kWh pack cell.
Samsung INR18650-35E (3500 mAh, 8 A) — current high-energy 18650.
Molicel P28A (2800 mAh, 35 A) — high-power 18650 for tools and racing.
LG INR21700-M50LT (5000 mAh, 7.3 A) — current high-energy 21700.
Samsung INR21700-50E (5000 mAh, 9.8 A) — humanoid robot default.
Molicel P42A (4200 mAh, 45 A) — high-power 21700, EV and high-discharge robotics.
Sony / Murata VTC6 (3000 mAh, 30 A) — vape industry but excellent power cell.

LiPo / LiHV pouch packs (drone class):

Tattu R-Line v5 (75–95C) — racing.
Auline / GAONENG GNB (60–120C HV) — racing.
Gens Ace (25–60C) — sport / cinematic.
CNHL (60–100C) — budget high-discharge.

LiFePO4 cells (industrial / AGV):

A123 Systems ANR26650M1B (2.5 Ah, 50 A continuous) — legacy high-power LFP.
EVE LF105 (105 Ah prismatic) — current industrial AMR standard.
EVE LF280K (280 Ah prismatic) — solar / large stationary, large AGV.
CATL LFP 105 / 280 — Chinese OEM equivalents at lower price.
BYD Blade (long thin prismatic) — automotive LFP; appearing in large mobile platforms.

NMC pouch:

CATL, BYD, LG Energy Solution, SK On, Samsung SDI, Panasonic. Available primarily through volume contracts; not catalog parts.

BMS ICs (cell monitoring):

TI BQ76952 (16S, with integrated FETs option) — current default for 4S–16S robotics packs.
TI BQ7693006 (10S, cost-optimised) — small packs.
TI BQ40Z80 (gas-gauge + protector for 7S laptop-class packs).
TI BQ79616 (16S automotive-qualified, daisy-chain).
Maxim / ADI MAX17853 (12S, distributed iso-SPI).
ADI LTC6811-1 / LTC6813-1 (12S / 18S, the EV / robotics distributed standard).
ADI LTC6820 (iso-SPI bridge to MCU).
NXP MC33771C / MC33772C (automotive, 14S / 6S).
Renesas ISL94203 (3S–8S standalone protector).

Off-the-shelf BMS modules (when a custom board is not justified):

Daly Smart BMS (4S–24S, 30–500 A) — Chinese, ubiquitous, CAN/Bluetooth.
JBD / JKBMS / OverKill — Chinese DIY-friendly, 4S–24S, Bluetooth + RS-485.
Orion BMS 2 (12S–48S) — US, robotics / EV-grade, CAN, ~$1000.
REC Active BMS (3S–28S) — European, marine / industrial.
Lithium Werks Valence U-BMS — paired with their LFP module packs.

DC-DC converters:

TI LM5170-Q1 — bidirectional multiphase buck-boost, ideal for hybrid pack-to-pack or pack-to-bus levelling.
TI TPS54xxx, LMR3xx series — small synchronous bucks for compute rails.
Vicor BCM / PRM / DCM — modular bricks, 100–400 W, sealed; mobile robot industry default for isolated rails.
Murata, Recom, RECOM RPP series — board-mount isolated bricks.
Meanwell RSD / DDR / SD families — DIN-rail, industrial AGV.

Motor drivers (BLDC FOC):

ODrive Robotics: ODrive S1 (single-axis, 80 V / 40 A peak, $250), O Dr i v e P ro (s in g l e - a x i s, 58 V /60 A p e ak, C A N / SP I,$ 399). Open hardware, mature firmware. Standard in research robotics and educational platforms.
VESC project: VESC 6 MKVI (75 V / 100 A), VESC 75/300 (75 V / 300 A peak — e-bike / e-foil class). Vedder open hardware + firmware; ubiquitous in e-mobility and converted to robotics widely.
Maxon: EPOS4 (50 V / 5–25 A; CAN, EtherCAT, USB; ~ $400) — co b o t - c l a ss; ESCONM o d u l e 50/8 (50 V /8 A co n t in u o u s,$ 300) for direct analog/PWM closed-loop drive.
Elmo Motion Control: Gold Twitter (15 A continuous, 30 A peak, EtherCAT, IP67) — cobot joints; Whistle (5–25 A, CAN/EtherCAT) — compact.
Synapticon: SOMANET Node 1000 — EtherCAT-native, integrated for cobot/AMR.
Trinamic / ADI: TMC4671 (FOC core IC paired with TMC6200 gate driver) — fully integrated single-IC FOC, used in cobots and 3D printers.
Roboteq / Nidec: HBL2350 / SBL series — AGV-class brushless controllers, 60 V / 25 A class.
TI driver ICs: DRV8323 (3-phase smart gate driver, 60 V class), DRV3245-Q1 (auto-qualified) — used inside custom inverter boards.
ST: L6230, L6234, STSPIN32 family — small low-end.
NXP: MC34GD3000 (3-phase pre-driver, 60 V).
MJBots: moteus controller r4.11 (48 V / 100 A peak, FD-CAN, fully open) — legged-robotics community default; QDD pair with moteus + 100 N·m planetary.
CubeMars / T-Motor: AK-series QDD actuators (motor + planetary + driver + encoder in one housing, CAN); AK80-9, AK60-6 used in MIT-derivative quadrupeds.

Power MOSFETs and gate drivers (for custom inverters):

Si MOSFET: Infineon OptiMOS / CoolMOS, Toshiba TPN-series, ON Semi NTMFS, Nexperia PSMN — 30 V to 200 V, R_DS(on) sub-mΩ. 100 V class for 48 V bus, 60 V class for 24 V bus.
SiC MOSFET: Wolfspeed C3M (1200 V class — autonomous vehicle traction, not for SELV bus), Infineon CoolSiC.
GaN HEMT: GaN Systems GS-065 series (650 V), EPC2218 (100 V — for 48 V SELV bus high-frequency drives, 1 MHz+). Use case in robotics: ultra-compact high-bandwidth FOC for ankle / wrist joints; not yet mainstream.
Gate drivers: TI UCC27xxx, Infineon EiceDRIVER, Si8xxx isolated drivers (Skyworks/Silicon Labs), ADuM4135 (ADI iso-driver).

Connectors (DC power):

XT60 (drone, 60 A, $0.50) — 4S/6S packs.
XT90, XT90-S (anti-spark, 90 A) — bigger drones, light mobile.
EC5, AS150 — high-power RC and small AGVs.
Anderson PowerPole (15–350 A) — mobile robot industry standard, hot-pluggable, gendered, polarity-keyed.
Anderson SB50 / SB175 / SB350 — AGV main power.
Amphenol PowerLok — EV-class, sealed, IP67.

Brake / dump resistors:

Ohmite TGHG (TO-220 / TO-247 thick-film, 50–250 W).
Vishay RWM / RFM wirewound (50 W–2 kW).
Custom nichrome ribbon on aluminium chassis for large dissipations.

Charging stations and chargers:

Meanwell RSP / HEP / HLG series — industrial CC-CV chargers, 24 V / 48 V outputs.
Delta-Q QuiQ — AGV / forklift class, programmable algorithms.
WiBotic 300 W and 3 kW wireless charging — autonomous dock alignment robots.
Mojo Mobility — wireless AGV docks.
Plugless / Continental — automotive wireless prototypes.

Cell-level fusing and protection:

Sinofuse, Schurter, Bourns BK fuse — per-string fuses inside packs.
Cell-level PTC (positive temp coefficient) or fusible-link tabs — common in laptop and EV packs.

6. Reference data

Chemistry comparison

Chemistry	V_nom / cell	V_max	V_min	Wh/kg (cell)	Wh/L	Cycles to 80 % SoH	Cost $/kWh (2025)	Safety
LiPo (NMC pouch)	3.7 V	4.2 V	3.0 V	180–260	350–700	300–500	250–400	Moderate — fire on abuse
LiHV	3.8 V	4.35 V	3.0 V	200–280	400–750	200–400	300–450	Moderate
Li-ion 18650	3.6–3.7 V	4.2 V	2.5–3.0 V	220–270	600–700	500–1000	130–200	Moderate, hardshell
Li-ion 21700	3.6–3.7 V	4.2 V	2.5–3.0 V	240–280	700–740	500–1500	100–180	Moderate, hardshell
LiFePO4	3.2 V	3.65 V	2.5 V	90–160	220–330	2000–6000	100–150	High — no thermal runaway typical
NMC pouch (large)	3.7 V	4.2 V	3.0 V	230–260	600–700	1000–2000	100–140	Moderate
Lead-acid (AGM)	2.0 V	2.4 V	1.75 V	30–40	80–90	300–800	100–150	High (vented), heavy
Solid-state (2026 dev)	3.7 V	4.2 V	3.0 V	350–500 (target)	800–1000 (target)	1000+ (target)	400+ (current)	Very high (target)

Standard pack configurations and energies

Config	V_nom	V_max	Typical capacity	Energy	Typical application
1S LiPo	3.7 V	4.2 V	0.3–1 Ah	1–4 Wh	Micro / Tiny Whoop
2S LiPo	7.4 V	8.4 V	0.5–2 Ah	4–15 Wh	2-inch racer
3S LiPo	11.1 V	12.6 V	1.5–3 Ah	17–33 Wh	3-inch racer, light hobby robot
4S LiPo	14.8 V	16.8 V	1.3–6 Ah	19–90 Wh	5–9-inch drone, RC car
6S LiPo	22.2 V	25.2 V	1.3–6 Ah	29–134 Wh	7–10-inch cinematic, RC truck
8S Li-ion (24 V class)	28.8 V	33.6 V	5–20 Ah	144–576 Wh	Small AMR, e-bike
13S Li-ion (48 V class)	46.8 V	54.6 V	10–30 Ah	468–1404 Wh	Cargo e-bike, light mobile robot
14S Li-ion (52 V class)	50.4 V	58.8 V	10–40 Ah	504–2016 Wh	E-bike, legged robot
16S LiFePO4 (48 V class)	51.2 V	58.4 V	100–300 Ah	5–15 kWh	Heavy AMR, off-grid, marine
20S Li-ion (72 V)	72 V	84 V	20–60 Ah	1.4–4.3 kWh	E-motorcycle, large AGV
96S NMC (350 V)	355 V	403 V	60–200 Ah	21–71 kWh	Autonomous vehicle, eVTOL

BMS IC comparison

IC	Cells	Topology	Bal. current (passive)	Comms	Notes
TI BQ7693006	6–10S	Centralised	8 mA on-chip	I²C	Cost-optimised
TI BQ76952	3–16S	Centralised	External	I²C / SPI	Robotics default
TI BQ40Z80	1–7S	Gas-gauge + protect	External	SMBus	Laptop class
TI BQ79616	6–16S	Distributed daisy	External	UART daisy	Auto-qualified
ADI LTC6811-1	12S	Distributed	60 mA / cell	iso-SPI	EV / heavy AGV standard
ADI LTC6813-1	18S	Distributed	60 mA / cell	iso-SPI	EV / heavy AGV standard
Maxim MAX17853	12S	Distributed	External	UART	EV class
NXP MC33771C	14S	Distributed	300 mA	TPL daisy	Automotive
NXP MC33772C	6S	Standalone	300 mA	SPI	Small auto packs
Renesas ISL94203	3–8S	Standalone	External	SMBus	Cost-optimised

BLDC driver families

Driver	Bus V	Current (cont. / peak)	Comms	Source	Use case
ODrive S1	12–58 V	40 A peak	CAN, USB	Open	Research, education
ODrive Pro	12–58 V	60 A peak	CAN, USB	Open	Research, robotics
VESC 6 MKVI	14–60 V	80 A / 200 A peak	CAN, USB, UART	Open	e-mobility, mobile robots
VESC 75/300	14–75 V	150 A / 300 A peak	CAN, USB	Open	High-power e-mobility
Maxon EPOS4 50/5	10–50 V	5 A / 15 A peak	CAN, EtherCAT, USB	Proprietary	Cobot, lab
Maxon ESCON 50/8	10–50 V	8 A / 30 A peak	Analog / PWM	Proprietary	Lab, OEM
Elmo Gold Twitter	12–95 V	15 A / 30 A peak	EtherCAT, CAN	Proprietary	Cobot joint
Synapticon Node 1000	12–48 V	25 A / 50 A peak	EtherCAT	Proprietary	Cobot / AMR
MJBots moteus r4.11	10–44 V	40 A / 100 A peak	FD-CAN	Open	Legged robotics
TMC4671 (custom inv.)	up to 75 V	board-dependent	SPI / step+dir	IC	Cobot, 3D printer
Roboteq HBL2350	10–60 V	25 A continuous	CAN, RS-232	Proprietary	AGV, mobile base

Brake resistor sizing reference (typical pack-to-resistor matches)

Bus V (nom)	Pack	Peak regen power	Resistor R	Resistor P_avg	Resistor P_peak	Example part
14.8 V	4S LiPo drone	100 W	4.7 Ω, 50 W	5 W	100 W	Vishay RFM50
22.2 V	6S LiPo	200 W	5.6 Ω, 100 W	10 W	200 W	Ohmite TGHG
25.6 V	8S LiFePO4 AMR	500 W	4.7 Ω, 250 W	50 W	500 W	Vishay RWM 250
51.2 V	16S LiFePO4 AMR	2 kW	8.0 Ω, 500 W	200 W	2 kW	Wirewound custom
48 V	13S Li-ion legged	1.5 kW	6.8 Ω, 500 W	100 W	1.5 kW	Ohmite L500J
72 V	20S Li-ion AGV	5 kW	22 Ω, 2 kW	500 W	5 kW	Wirewound bank

Charge connectors and standards

Connector	Typical V	Typical I	Standard	Use case
Anderson SB50	24–48 V	50 A	proprietary	AGV dock
Anderson SB175	48–80 V	175 A	proprietary	AGV / forklift dock
WiBotic Tx/Rx coil	24–48 V	12–60 A equivalent	proprietary	Wireless AMR dock
J1772 Type 1	240 V AC	32–80 A	SAE J1772	EV (and EV-class robot)
CCS Type 1	up to 1000 V DC	up to 500 A	IEC 62196-3	EV DC fast charge
GB/T 20234	750 V DC	250 A	China DC fast	Chinese-market autonomy
NACS / Tesla	1000 V DC	500 A	SAE J3400 (2023)	Tesla supercharger + adopting OEMs
USB-PD 3.1 EPR	28–48 V	up to 5 A (240 W)	USB-IF	Small mobile robots, drones (Mavic 3)

Cell vendor + spec quick reference

Cell	Capacity	Max cont. discharge	Mass	Wh/kg	Vendor
NCR18650B	3400 mAh	4.9 A	47.5 g	257	Panasonic
INR18650-35E	3500 mAh	8 A	49 g	257	Samsung SDI
P28A	2800 mAh	35 A	46 g	219	Molicel
M50LT	5000 mAh	7.3 A	70 g	264	LG ES
INR21700-50E	5000 mAh	9.8 A	69.5 g	266	Samsung SDI
P42A	4200 mAh	45 A	70 g	222	Molicel
VTC6	3000 mAh	30 A	46.6 g	238	Sony / Murata
LF105 (LFP)	105 Ah	1C cont.	1.96 kg	171 (Wh/kg pack-level lower)	EVE
LF280K (LFP)	280 Ah	1C cont.	5.4 kg	166	EVE
ANR26650M1B (LFP)	2.5 Ah	50 A	76 g	109	A123

7. Failure modes & debugging

Battery swelling. Pouch LiPo or NMC cells that have absorbed water or overgassed will visibly puff. Retire immediately; gas pressure raises puncture risk. Never recharge a swollen pack. Disposal: 0 V drain via brake resistor, then to certified Li-ion recycling (Call2Recycle in NA, Stiftung GRS in EU). Symptom: pack thickness exceeds spec by > 5 %, weight unchanged.

Cell imbalance during charge. A pack that finishes charge at, say, 4.20 / 4.20 / 4.18 / 4.05 V across a 4S string has a weak cell 4 that is not fully charged because the BMS terminated when the strongest cell hit V_max. Usable energy is set by the weakest cell. Causes: manufacturing variation, age, temperature gradient inside the pack. Fix: run a balance charger (iCharger, ISDT) overnight at 0.1C until all cells equalise. If imbalance recurs within a few cycles, the weak cell is failing — replace the pack (rarely worth rebuilding).

Over-current trip on aggressive acceleration. The ESC or BMS opens because phase or pack current exceeded the trip threshold. First check: is the motor commutating correctly (encoder alignment, see [[Robotics/motors-electric]] §7), or is it stalling and pulling lock-rotor current? Second: is the C-rate of the pack adequate (continuous and burst)? Third: is the BMS trip set above the worst-case acceleration current with margin? Fourth: are the FOC current loop gains overshooting?

Voltage sag under load larger than expected. Two causes: (a) pack R_int higher than spec — cold cells, aged cells, counterfeit cells. Measure R_int directly with a 1 kHz AC milliohm meter or a current-step test. (b) Wiring + connector resistance — a 30 cm of 12 AWG at 100 A drops 0.55 V; that’s free voltage loss the BMS doesn’t know about. Upgrade conductors and clean contacts.

Cold-weather capacity loss. At 0 °C a Li-ion cell delivers ≈ 70 % of its 25 °C capacity. At −20 °C, ≈ 40 %. Critical: do not charge below 0 °C — lithium plates on the anode and the cell can short internally. Mitigations: pre-heat with a low-current resistive heater integrated in the pack, or a stay-in-dock heater. Industrial cold-store AGVs use insulated pack jackets with a 25 W heater driven from the bus when SoC > 50 %.

Thermal runaway propagation. A single cell failing (internal short, external puncture) can hit 200+ °C and ignite neighbours. Mitigations: physical cell spacing (typical pouch-pack 1–3 mm with intumescent foam), aluminium busbars sized to conduct heat out of the pack, and ceramic / mica thermal barriers. Required for any pack > 2 kWh.

BMS comms failure. Host controller reports loss of CAN frames or stale SoC. Almost always cabling: chafed CAN wires, missing 120 Ω termination at both ends of the bus, ground-loop noise from the motor inverter contaminating the BMS signal ground. Re-do termination, route the BMS bus away from the motor phase wires (≥ 100 mm spacing or shielded twisted pair), check single-point ground.

DC bus ripple and motor noise. Inverter switching creates 20 kHz–1 MHz ripple on the bus; if the bus capacitance is too small or its ESR too high, the current loop sees voltage perturbations and the joint torque is noisy. Add bus capacitance (low-ESR polymer 1000–4700 µF) and an input LC filter on every distributed inverter. See [[Engineering/power-electronics]] §EMI for the design.

Ground loops in distributed compute. Multiple ground returns through chassis-bonded compute, sensors, and motor drivers create voltage offsets that look like sensor drift, jittery encoder counts, or false E-stop trips. Single-point bond at the battery negative; isolated CAN (CTM transceivers with ADuM1201 isolators) between compute and motor domains.

Inverter short or destruction. A bridge short on a BLDC driver typically takes out at least one MOSFET pair and often the gate driver. Causes: shoot-through from inadequate dead-time, voltage spike from cable inductance during fast switching without proper snubber, water ingress. Always size phase fuses or use desat-detecting drivers (DESAT comparators in industrial gate drivers).

Regen overvoltage / bus pump. The bus voltage spikes above OVP threshold on hard deceleration and the drive trips. Cause: brake resistor undersized or absent, or chopper threshold mis-set, or battery cannot accept current (full or LFP at low temperature). Fix: re-size brake resistor for worst-case regen power, lower chopper engage voltage (e.g., engage at V_bus = 1.10 · V_nom rather than 1.20).

Counterfeit cells. Particularly via online retailers, “Panasonic NCR18650B” can ship with 40–60 % of nameplate capacity. Detect: weight (47 g real; light = fake), discharge capacity to 2.5 V at 1A, internal resistance at full charge. Buy from authorised distributors (Digi-Key, Mouser, IPP, 18650BatteryStore) only.

Charger over-spec / under-spec. A charger that delivers above-spec voltage damages cells; a charger that terminates current incorrectly (no CV phase, or too-high termination current) leaves cells under-charged. Verify with a calibrated DC analyser (RIGOL DL3021, Maynuo, EBC-A40L) before commissioning.

Aging beyond SoH 80 %. Pack capacity has degraded enough that runtime is unacceptable and voltage sag triggers trips. No “fix” — replace the pack. Track SoH in the BMS log; schedule replacement on calendar plus condition.

Fire response. A Li-ion fire is not extinguishable with water in any practical sense. Mitigate spread: a Class D extinguisher or sand for small packs; for large packs, isolation and burn-out under containment. Workplace robotics charging stations should be in fire-rated enclosures with smoke detection routed to facility fire panel.

Inverter desat / DESAT trip. Industrial gate drivers (Skyworks Si827x, ADI ADuM4135, TI UCC21750) monitor V_CE / V_DS during the ON state; if it exceeds a programmable threshold (e.g., 7 V), they assume the device is in saturation under fault and force a soft turn-off to avoid latch-up. A DESAT trip during normal operation usually points to a faulty motor (shorted turn), bad encoder commutation (wrong angle pushing the device into linear region), or a damaged MOSFET with rising R_DS(on). Always check the gate-driver fault log before reflashing firmware.

Encoder commutation misalignment. A BLDC under FOC requires the rotor angle to within ±5° electrical for efficient torque production; ±15° and the motor heats but still spins; ±30° and the current loop oscillates and the inverter trips. Whenever a motor “draws too much current for the load it’s delivering,” check encoder-to-rotor alignment first via the driver’s calibration routine (ODrive motor.calibrate, VESC Motor Setup Wizard).

Inrush damage to compute rails. Closing the main contactor without pre-charge, or hot-plugging a 12 V rail to a Jetson while the pack is energised, can blow the compute’s input protection diode. Always pre-charge; on-board input protection should include a TVS diode rated to the contactor closure transient (typically a 1.5KE-class TVS at 1.5× nominal rail).

8. Case studies

Tesla Optimus battery (Gen 2, 2024)

Tesla disclosed at AI Day 2024 that the Optimus humanoid carries a 2.3 kWh pack at a 48 V nominal bus, integrated in the torso. Cells are Tesla 21700 (NCA chemistry, similar to Model 3 short-range), 12S configuration with ≈ 60 cells parallel per series-group for a total ~720 cells. The pack runs the 28-DOF actuator array — quasi-direct-drive joints for the legs (Tesla custom design analogous to MJBots-class), Harmonic Drive geared joints for the fingers. Charge time approximately 2 hours from depleted via a custom dock. Continuous workload duration ~22 minutes; this is published as the next generation target, with Gen 3 promising 60+ minutes. Notable detail: Tesla integrates the BMS directly into the pack PCB rather than a separate module, with iso-SPI distributed monitoring per ADI LTC6813.

Reference: Tesla AI Day 2024 disclosure; teardown analyses (Munro Live, 2024).

MiR250 / MiR500 AMR (Mobile Industrial Robots, A/S, Universal Robots subsidiary)

The MiR250 (250 kg payload, indoor warehouse) carries a 41 Ah / 24 V LiFePO4 pack (~ 1.05 kWh), 8S configuration with prismatic LF50 / similar cells. Runtime in typical fetch-and-carry duty cycle: 10 hours. Fast charge 1.5 hour from 10 % to 90 %. The dock is a brush-contact charger with a fiducial-marker-guided final-cm alignment via the front depth camera. Pack is certified to IEC 62619 and UL 1973; the chemistry choice is driven by unattended workplace operation — LiFePO4’s thermal-runaway resistance is the deciding factor over higher-density NMC. The MiR500 (500 kg payload) carries a larger 56 Ah / 48 V LiFePO4 pack ( 2.7 kWh).

Reference: MiR250 product datasheet (current as of 2026 catalogue); MiR Charge Station 24/48 V manual.

Unitree Go2 quadruped pack and QDD bus (2024)

The Unitree Go2 quadruped runs a 12S Li-ion pack at 8000 mAh, ~44.4 V nominal, ~360 Wh total, mass ~1.6 kg, providing roughly 2.5 hours of nominal walking or ~1 hour of dynamic running. Twelve quasi-direct-drive joints (M107 and similar) draw peak phase currents up to 30 A each during gait disturbances, with mean per-joint power around 25–60 W in level walking. The bus is a single 48 V class DC rail feeding all twelve integrated joint drivers (FOC-controlled, FD-CAN feedback at 1 kHz), with a small isolated DC-DC for the onboard NX-class compute. Critically, the pack accepts regen during touchdown shock: at heel-strike, a leg’s motor briefly operates as a generator and pumps several joules back into the bus over ~50 ms. Without a brake chopper, this would trip the inverter on cold-pack mornings; Unitree includes a small bus-clamp circuit and sufficient bus capacitance to absorb the per-step regen pulses. The pack is hot-swappable from a side bay — a deliberate design choice to enable continuous operation in field deployments.

Reference: Unitree Go2 technical specification, Unitree Robotics, 2024 catalogue; MJBots moteus and reverse-engineering writeups of Go1/Go2 architecture (community-published).

DJI Mavic 3 Intelligent Flight Battery (BWX260-5000-15.4)

The DJI Mavic 3 ships with a 5000 mAh / 15.4 V LiPo (4S; nominal 14.8 V, full 16.8 V) pack rated 77 Wh, mass 335 g, enabling 46 minutes of flight time (advertised; real-world 35–40 min). The intelligent BMS monitors per-cell voltage, pack temperature, charge cycles, and pre-flight self-test. Communication to the aircraft is over a proprietary serial bus carrying SoC, SoH, temperature, current, voltage, and cycle count. Type-C bidirectional charging at up to 100 W means a full charge in ~95 minutes; “Intelligent Discharge” auto-self-discharges to 60 % over a configurable number of idle days to extend calendar life — a feature absent from most non-DJI drone batteries. The battery’s BMS will refuse to allow flight if any cell is < 3.6 V or pack temperature is outside −10 °C to 40 °C.

Reference: DJI Mavic 3 specifications (current catalog); DJI BWX260 service manual.

8b. Worked discharge / regen budget — a tracked AMR going downhill

A 100 kg outdoor AMR with 48 V LiFePO4 pack (16S, 100 Ah = 5.12 kWh). The robot descends a 5 ° ramp at 1 m/s, travelling for 30 m. The gravitational potential energy released:

ΔPE = m · g · h = 100 · 9.81 · (30 · sin 5°) = 100 · 9.81 · 2.62 = 2570 J.

Travel time: 30 m / 1 m/s = 30 s. Average regen power: 2570 / 30 = 86 W. Modest — the battery can absorb this directly at any SoC below ~95 %. No brake resistor engages.

Now consider an emergency stop from 1.5 m/s on the same ramp: kinetic energy ½ · 100 · 1.5² = 113 J dissipated in (say) 0.3 s gives 376 W; add 86 W gravity term, total regen 460 W over 0.3 s. Still within battery acceptance for a < 90 % SoC pack. But at 100 % SoC, the BMS will not accept the current: the bus voltage spikes, the brake chopper engages, and the 460 W (~140 J) is dumped into the brake resistor. With a 16 Ω, 500 W resistor and 50 V bus the chopper duty is (460 / (50² / 16)) = 460 / 156 = limited; in practice the chopper PWM-modulates onto the resistor to clamp the bus at, e.g., 56 V (full-pack OVP threshold). The 140 J is absorbed in 0.3 s; resistor temperature rise (specific heat 460 J/kg for steel-wound, mass 0.5 kg) = 140 / (460 · 0.5) = 0.6 K per event. Acceptable for repeated stops; the resistor’s continuous rating (500 W) handles the heat soak over a duty cycle of one stop per ~10 s.

If a future variant of this AMR carries cargo to 250 kg, the same ramp emergency-stop scales to 1.15 kW and the resistor needs upsizing. Always re-validate the regen path when the kinematic mass envelope changes.

8c. Power-electronics block diagram for a typical 48 V mobile robot

[16S LFP pack 51 V] ── HRC fuse ── precharge R ── main contactor ── busbar
                                                                 │
                                                       ┌─────────┼─────────┐
                                                       │         │         │
                                                  [3-ph inverter]  [3-ph inv.]
                                                  │ (left wheel)│  │(right whl)│
                                                  └──── BLDC ───┘  └─── BLDC ──┘
                                                       │         │
                                                       │         └──── brake R ──── chopper switch
                                                       │
                                                  [DC-DC buck 51 V → 24 V] ──── lights, lift actuator
                                                  [DC-DC buck 51 V → 19 V] ──── Jetson compute (isolated)
                                                  [DC-DC buck 51 V → 12 V] ──── sensors, fans
                                                  [DC-DC buck 51 V →  5 V] ──── logic, microcontroller
 
[BMS, distributed cell-monitor daisy] ──── iso-CAN ──── main controller
[safety relay] ──── E-stop chain ──── main contactor coil
[charge port (Anderson SB50)] ──── isolation diode + charger ──── BMS

Each branch carries its own fuse sized for 1.25 × maximum continuous load; the safety relay opens the main contactor independently of any software path. Iso-CAN (transceiver-isolated CAN) keeps motor-domain switching noise from corrupting BMS telemetry.

9. Cross-references

[[Robotics/motors-electric]] — the motor + driver chain that the bus feeds. Power-system bus voltage and driver pick are inseparable from motor sizing.
[[Robotics/multirotor-design]] — companion in the same batch; drone-specific propulsion + power loop.
[[Robotics/mobile-base-wheeled]] — wheeled mobile platform geometry and drivetrain.
[[Robotics/bayesian-estimation]] — SoC / SoH estimation via Kalman / EKF.
[[Robotics/computer-vision-robotics]] — dock-alignment perception for autonomous charging.
[[Engineering/power-electronics]] — buck / boost / inverter theory and EMC.
[[Engineering/electric-motors]] — motor torque-speed and back-EMF foundations.
[[Engineering/semiconductor-devices]] — Si / SiC / GaN switch characteristics, R_DS(on), gate charge.
[[Engineering/op-amps]] — current-sense amplifier design for low-side and high-side shunts.
[[Engineering/transformers-power-systems]] — isolated charger front-ends.
[[Languages/Tier3/robotics-control]] — controller description languages.

10. Citations

Plett, G. L. Battery Management Systems, Volumes I (Battery Modeling) and II (Equivalent-Circuit Methods). Artech House, 2015–2016.
Rahn, C. D. and Wang, C.-Y. Battery Systems Engineering. Wiley, 2013.
Linden, D. and Reddy, T. B. (eds.) Linden’s Handbook of Batteries, 5th ed. McGraw-Hill, 2024.
Krause, P. C., Wasynczuk, O., and Sudhoff, S. D. Analysis of Electric Machinery and Drive Systems, 4th ed. Wiley-IEEE Press, 2024.
Erickson, R. W. and Maksimović, D. Fundamentals of Power Electronics, 3rd ed. Springer, 2020.
Mohan, N., Undeland, T. M., and Robbins, W. P. Power Electronics: Converters, Applications, and Design, 3rd ed. Wiley, 2002 (still the canonical industry textbook for inverter design).
IEC 62133-2:2017 — Secondary cells and batteries containing alkaline or other non-acid electrolytes — Safety requirements for portable sealed secondary lithium cells.
IEC 62619:2022 — Secondary cells and batteries containing alkaline or other non-acid electrolytes — Safety requirements for secondary lithium cells and batteries, for use in industrial applications.
UL 2580:2020 — Standard for Batteries for Use in Electric Vehicles.
UN Manual of Tests and Criteria, Part III, Section 38.3 — Lithium metal and lithium-ion batteries (UN 38.3 transport testing).
SAE J1772:2017 — SAE Electric Vehicle and Plug-in Hybrid Electric Vehicle Conductive Charge Coupler.
ISO 13849-1:2023 — Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design.
Tesla AI Day 2024 — Optimus disclosure (recorded keynote, October 2024).
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Appendix A — Quick-reference C-rate and runtime estimator

For a pack of capacity C [Ah] at nominal voltage V [V] delivering an average current I [A]:

runtime [h] ≈ (C · η_usable) / I

with η_usable typically 0.85 for LiPo (knee + safety), 0.90 for Li-ion, 0.95 for LiFePO4 (flatter discharge curve).

Discharge time at constant C-rate: t [min] = 60 / C-rate. Equivalent C-rate at current I: C-rate = I / C_Ah.

Pack mass approximation: m_pack [kg] ≈ E_pack [Wh] / (Wh/kg from §6 chemistry table), then multiply by 1.2–1.4 for casing, BMS, busbars, and connectors (the pack-level Wh/kg is always lower than cell-level Wh/kg).

Appendix B — Common derating tables

Variable	Derate from nameplate by
Continuous current rating	80 % for thermal margin
Burst current	90 % for connector / fuse safety
Cell C-rate (vendor spec)	70 % for engineering work
Battery capacity end-of-life	80 % SoH planning
MOSFET R_DS(on)	× 1.5–2× from 25 °C to T_j_max
DC-DC efficiency	published peak is at one operating point; assume −5 % typical
Wire ampacity (chassis-mounted bundled)	× 0.7 vs free-air
Charger output (continuous over ambient 40 °C+)	× 0.9

Session log entry was appended at the conclusion of this note write.

Compendium

Explorer

Robot Power Systems — Batteries, BLDC Drivers, Regen — Robotics Reference

Robot Power Systems — Batteries, BLDC Drivers, Regen

1. At a glance

2. First principles

3. Practical math — three worked sizing examples

Example A — Indoor AMR battery sizing

Example B — Drone discharge profile (continued from `[[Robotics/motors-electric]]` Example C)

Example C — BLDC current loop sizing

4. Design heuristics

5. Components & sourcing

6. Reference data

Chemistry comparison

Standard pack configurations and energies

BMS IC comparison

BLDC driver families

Brake resistor sizing reference (typical pack-to-resistor matches)

Charge connectors and standards

Cell vendor + spec quick reference

7. Failure modes & debugging

8. Case studies

Tesla Optimus battery (Gen 2, 2024)

MiR250 / MiR500 AMR (Mobile Industrial Robots, A/S, Universal Robots subsidiary)

Unitree Go2 quadruped pack and QDD bus (2024)

DJI Mavic 3 Intelligent Flight Battery (BWX260-5000-15.4)

8b. Worked discharge / regen budget — a tracked AMR going downhill

8c. Power-electronics block diagram for a typical 48 V mobile robot

9. Cross-references

10. Citations

Appendix A — Quick-reference C-rate and runtime estimator

Appendix B — Common derating tables

Graph View

Table of Contents

Compendium

Explorer

Robot Power Systems — Batteries, BLDC Drivers, Regen — Robotics Reference

Robot Power Systems — Batteries, BLDC Drivers, Regen

1. At a glance

2. First principles

3. Practical math — three worked sizing examples

Example A — Indoor AMR battery sizing

Example B — Drone discharge profile (continued from [[Robotics/motors-electric]] Example C)

Example C — BLDC current loop sizing

4. Design heuristics

5. Components & sourcing

6. Reference data

Chemistry comparison

Standard pack configurations and energies

BMS IC comparison

BLDC driver families

Brake resistor sizing reference (typical pack-to-resistor matches)

Charge connectors and standards

Cell vendor + spec quick reference

7. Failure modes & debugging

8. Case studies

Tesla Optimus battery (Gen 2, 2024)

MiR250 / MiR500 AMR (Mobile Industrial Robots, A/S, Universal Robots subsidiary)

Unitree Go2 quadruped pack and QDD bus (2024)

DJI Mavic 3 Intelligent Flight Battery (BWX260-5000-15.4)

8b. Worked discharge / regen budget — a tracked AMR going downhill

8c. Power-electronics block diagram for a typical 48 V mobile robot

9. Cross-references

10. Citations

Appendix A — Quick-reference C-rate and runtime estimator

Appendix B — Common derating tables

Graph View

Table of Contents

Example B — Drone discharge profile (continued from `[[Robotics/motors-electric]]` Example C)